Variable resistor



June 13, 1950 Y E, PACKARD 2,511,807

VARIABLE RESISTOR Filed Jan. 26, 1949 [liv/@aaa 2 Patented June 13, 1950 VARIABLE RESISTOR Lucius E. Packard, Waltham, Mass., assigner to Technology Instrument Corporation, Waltham, Mass., a corporation of Massachusetts Application January 26, 1949, Serial No. 72,790

3 Claims.

The present invention relates to potentiometers. variable resistors or voltage dividers, and more particularly to a unique wire wound resistance element for such devices.

Heretofore, linear ,and non-linear adjustable resistors or potentiometers have been available but the resistance characteristics thereof have been limited. Not infrequently it would be desirable to have a potentiometer or adjustable resistor having a resistance characteristic which not only is non-linear but which has a curve when plotted that is of the type commonly referred to as a logarithmic curve. Thus far only a portion of such curve could be embodied in a potentiometer or resistor but in accordance with the present invention it is possible to obtain a resistance element which has a derivative of resistance curve of the logarithmic type. This is accomplished by providing a form for the wound wire element which has a contour of stepped configurations symmetrical to the center line along which the adjustable contact arm will move.

It, therefore, is an object of the present invention to provide an improved construction of a resistance element for wire wound adjustable resistors for potentiometers.

Still another object of the present invention is to provide an improved adjustable resistor or potentiometer of the wire wound type which has a non-linear resistance characteristic having a.

high degree of accuracy.

Other and further objects oi' the present invention subsequently will become apparent by reference to the following description taken in conjunction with the accompanying drawings wherein:

Fig. l is a perspective view of a potentiometer constructed in accordance with the present invention;

Fig. 2 is a plan view of the insulation form which supports the wire elements of the resistor; and

Fig. 3 is a curve illustrating the characteristic of the potentiometer.

In Fig. l there is illustrated a potentiometer having a suitable casing II which is provided with a plurality of contacts or binding posts I2 and I3. The electrical connections or contacts I2 ,are connected to opposite ends of the wire wound resistor elements I4. The other electrical contact or binding post I3 1s connected to a twobrush rotor assembly which has an adjustable contact I6 which may be moved to the desired position upon the resistor Il.

Fig. 2 shows a form I1 made oi' a strip of insulating material for supporting the Wire resistance elements. The form I1 between the ends IB and I9 may have a plurality of portions so shaped as to give different characteristics to the wire element Wound thereon. Thus the portion 20 may provide a linear characteristic whereas the remaining portions 2|, 22 and 23 will provide non-linear resistance characteristics. It will be noted that the form I1 has a generally sawtooth configuration so that on opposite sides of the center line C the strip I1 is symmetrically arranged. In one form of a, particular resistance characteristic the portions 20 and 2I of the strip I1 were wound with a continuous resistance wire of a particular value of resistance per lineal unit. The next succeeding portion 22 was wound with a resistance wire having a higher resistance value and likewise the next succeeding portion 23 was wound with still another resistance wire having an even higher resistance characteristic.

Fig. 3 illustrates the derivative of resistance curve which shows the relation between the shaft rotation of the member which carries the adjustable contact I6 of Fig. 1 and the percent of func tion of the resistance value which is to be varied. It thus becomes apparent that the rate of change of resistance may be said to have a logarithmic characteristic.

While for the purpose of illustrating the invention Fig. 2 shows a strip for carrying the wire element which has three stepped portions 2|, 22 and 23, it of course will be appreciated that the invention is not to be limited thereby since a lesser or greater number of steps may be provided. By arranging the stepped coniiguration above and below the center line C it is possible to obtain any one of a number of desired characteristics. Thus it will also be appreciated that Fig. 3 merely illustrates one type of characteristic curve which can be obtained by employing a resistor strip construction which embodies the principles employed in forming the form I1 of Fig. 2. With a construction of this type it has been possible to provide precision potentiometers having an accuracy of one percent of the total resistance. The accuracy with which the curve is followed when the adjustable contact I6 is rotated may be as high as two-tenths of one percent of the total resistance. The attainment of this accuracy is attributed largely to the manner of constructing and employing the resistance form I'l of Fig. 2.

While for the purpose of illustrating and describing the present invention certain embodiments have been shown in the drawings. it is to be understood that the invention is not to be limited thereby since such variations are contemplated as may be taught by the invention and are set forth with particularity in the accompanying claims.

What I desire to protect by United States Letters Patent is claimed as follows:

l. In an adjustable resistor, a resistance element comprising a at sheet of insulation having opposite edges arranged in generally sawtoothed Configuration symmetrically relative to the center line thereof, opposite edges of at least one of said sawtoothed configurations having curved wire supporting configurations, each succeeding pair of sawtoothed edges carrying a resistance Wire of higher resistance value than the wire on the preceding sawtoothed edge.

2. An adjustable resistor of the wire wound type having a characteristic such that the derivative of resistance for the uniform rotation of a contact arm may be indicated graphically by a logarithmic type curve, comprising a casing, a rotatable contact arm. a wire wound resistor mounted in said casing so that the contact arm moves in a path along the center line of said resistor, said resistor including a Wire supporting form having generally saw-toothed edges arranged symmetrically relative to its center line, one pair of said saw-toothed edges having linear sides, another pair of said saw-tcothed edges f having curvilinear sides, each pair of opposed saw-toothed edges carrying a resistance wire of higher unit resistance value than the preceding one.

3. A variable resistor of the wire wound type having for uniform angular adjustment of the contact arm a characteristic such that the derivative of resistance follows a non-linear curve comprising a round casing, a rotatable contact arm and a Wire wound resistor mounted in said casing so that the contact arm moves in a path along the center line of said resistor, said resistor including a wire supporting strip of sheet insulating material having generally rip sawtoothed edges arranged symmetrically relative to its center line, each pair of saw-teeth carrying a wire having a different selected resistance value per unit length, said wire being positioned on the inclinededges of the saw-teeth so that each incremental change of position of the contact arm produces a non-linear incremental change of the total resistance between the contact arm and one end of said resistor.

LUCIUS E. PACKARD.

REFERENCES `CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,755,314 Carter Apir` 22, 1930 2,428,093 Raymond Sept. 30, 1947 FOREIGN PATENTS Number Country Date 377,315 Great Britain July 28, 1932 397,541 Great Britain Aug. 28, 1933 

